The present invention relates to a signal processing circuit for FM signal receiver, and more particularly to a combined circuit of a tuning level indication, a muting operation and an AFC (Auto Frequency Control) signal information.
In addition to the fundamental construction for receiving an FM signal, such as a tunner, a radio-frequency (RF) amplifier, a mixer, an intermediate-frequency (IF) amplifier, an FM demodulator, an audio-frequency (AF) and power amplifiers, and a loud speaker, recently FM radio receivers generally provide a tuning indicator for aiding a listener to select a broadcasting station, a level muting function which attenuates or interrupts an output when there is no received signal or when a received signal level is so low that good receiving characteristics cannot be obtained, a band muting function which attenuates or interrupts an output when precise tuning to a receiving signal frequency is not attained and hence a good demodulated output cannot be obtained, and/or an AFC function which automatically reduces variation of an oscillation frequency of a local oscillator due to variations of an environmental temperature and a power supply voltage. Consequently, an FM receiver requires many circuits for achieving the associated functions such as a drive circuit for a tuning indicator, a signal strength detector circuit, a frequency deviation detector circuit for detecting whether tuning is made precisely or not, a muting control circuit responsive to the outputs of the signal strength detector circuit and the frequency deviation detector circuit, for controlling a muting operation of the receiver, an output control circuit responsive to the output of the muting control circuit for deriving the output of the FM receiver or cutting off the output by attenuating or interrupting the output under the action of muting, an AFC signal deriving circuit, etc.
In the conventional FM radio receiver, the signal strength was detected from IF amplifier stage and indicated by an indicator such as moving coil type ammeter, as taught in U.S. Pat. No. 3,673,499. The listener could detect the precise tuning condition from the indication of the signal strength indicator. The muting operation was made by the peak-to-peak detection output from the angle modulation detector, and AFC operation by the output derived from the output amplifier, as taught in U.S. Pat. No. 3,714,583. Thus, each operation was made by the individual circuit associated to the individual fundamental circuit block, and so the conventional FM radio receiver had a disadvantage that its construction was extremely complex.